1. Field of the Invention
The present invention relates to an improved planar inverted-F antenna (PIFA), and in particular to a capacitive feed planar inverted-F multi-band antenna.
2. Description of Background Information
Antenna is an essential part of a wireless device. Over the years, wireless devices have been rapidly miniaturizing, thus increasing demand for integrated or built-in antennas. Concurrently, there has been an influx of wireless services and users. To cope with increasing usage and demand, many wireless devices and networks have since migrated from single band operation to dual band (or multi-band) operation to improve network capacity and coverage, and to provide users with seamless quality service.
A common integrated antenna used in wireless devices is the Planar Inverted-F Antenna (PIFA). The PIFA is a widely favored integrated antenna because it provides for a more compact antenna with an approximate length of xcex/4, which is an improvement over a length of xcex/2. A typical PIFA is shown in FIG. 1. The PIFA structure shown has a planar radiating element characterized by slits for defining two lips or length portions. Each lip corresponds to a resonant frequency at which the antenna operates. The radiating element has a feed point for directly connecting the radiating element to an antenna feed, and a short circuit point for connecting the radiating element to a ground element arranged below the radiating element. The described antenna structure of FIG. 1 is commonly known as a direct feed PIFA.
The direct feed PIFA is easy to design and fabricate, but its main disadvantage is insufficient bandwidth to support multi-band operation. Accordingly, there is a need to improve antenna performance by increasing bandwidth of a multi-band antenna while providing for a smaller form factor.
The present invention provides an integrated capacitive feed planar inverted-F antenna (PIFA) for multi-band operation. A typical embodiment of the present invention comprises a ground element, and a main radiating element arranged at a predetermined height from the ground element, the main radiating element having slits for defining lips. At one end of the main radiating element, it is short-circuited to the ground element. A feed element is arranged in the vertical gap between the ground and the main radiating elements. The feed element is detached (or separated by a gap) from the ground and main radiating elements to create capacitive feeding. For efficient feeding, the feed element may be arranged substantially parallel to the main radiating element. The invention also comprises an antenna feed which is electrically connected to the feed element, but detached from the main radiating and ground elements.
Secondary (or sub-radiating) elements may also be arranged in the vertical gap and proximate to the feed element for creating an additional resonant frequency or for improving bandwidth performance. The secondary elements are detached (or separated by a gap) from the main radiating, feed and ground elements.